This invention relates to the measurement of the electrical resistance per unit length of a filament, and finds particular application in situations in which it is inconvenient or impossible to make direct electrical contact with the conductive filament. A particular example of an instance where direct electrical contact is either undesirable or impossible concerns making measurements of electrical resistance per unit length of electrically conductive coatings, for instance carbon coatings, applied to optical fibres. In this instance, direct contact with the coating may be undesirable for risk of damaging the strength of the fibre, or because the electrically conductive coating has already itself been coated with a dielectric protective coating, for instance, of plastics material.
During the manufacture of an optical fibre with a hermetic coating of carbon, it is desirable that the thickness of the carbon coating is monitored. One approach is to measure the conductance of the fibre, which, for thin coatings of conductivity .sigma., is proportional to the thickness t. EQU conductance per unit length=2.pi.at.sigma. (t&lt;&lt;a) (1)
where a is the fibre radius.
FIG. 1 depicts how an existing design of tensometer can be adapted to enable the making of direct current on-line measurement of electrical resistance per unit length. In this tensometer, the optical fibre 10 passes over two guide wheels 11 and 12 between which the fibre is deflected by a resiliently biased tensometer wheel 13. The tensometer is adapted by replacing the original wheels 11 and 12 with graphite wheels, and by connecting a resistance meter 14 across the graphite wheels using sliding contacts 15. As indicated previously, the drawback of this approach is that the direct contact between the carbon coating and the tensometer wheels is liable to damage the coating and hence impair the strength of the fibre, whereas, if the carbon is provided with a plastics protective coating prior to passing through the tensometer, the presence of this protective coating prevents direct electrical contact between the carbon coating and the graphite wheels.
If, instead of using direct current to make the measure, alternating current were to be used, then at least in principle, contactless capacitive coupling can replace the graphite wheels of FIG. 1. However, in practice the fibre will also be susceptible to capacitive coupling with other parts of the fibre drawing equipment, as indicated schematically in FIG. 2 and in the equivalent circuit diagram of FIG. 3. In FIGS. 2 and 3, Z.sub.A and Z.sub.B are largely capacitive impedances, the product of interaction with the surrounding equipment of the fibre either side of the coupling capacitances C.sub.in and C.sub.out. Both the potential induced on the fibre, V.sub.f, derived from an oscillator 20 providing an a.c. signal V.sub.i, and the fraction of V.sub.f tapped out to give the output signal, V.sub.1, depend on the magnitudes of the stray impedances Z.sub.A and Z.sub.B.
In order to calculate the fibre resistance R unambiguously, it is necessary either to know the stray impedances Z.sub.A and Z.sub.B, or to use multiple probes to derive both the potential on the fibre V.sub.f and its rate of change. For this purpose, two capacitive taps, C.sub.1 and C.sub.2, respectively tapping off signals V.sub.1 and V.sub.2 fed to buffer amplifiers 40, may be employed as indicated in FIG. 4. Some form of screening 41 between the oscillator and the taps will also be necessary to reduce direct capacitive coupling between the oscillator and the taps. The equivalent circuit is illustrated in FIG. 5.